This invention relates to a telemeter system for measuring information relating to e.g. ocean water such as temperature or salinity at various depths, and more particularly to a receiving system for such a telemeter system using a measuring probe for radiating an ultrasonic pulse signal. An ultrasonic signal is advantageous because it can be transmitted in water with low loss.
There are various methods for modulating the carrier of an ultrasonic wave in such a telemeter system, and from the standpoint of effective use of transmitting power and simplification of the system, it is advantageous to pulse modulate the ultrasonic signal so as to make the pulse spacing of the sound pulse signal correspond to information to be measured. The inventors have proposed an expendable oceanography probe which can be effectively used in such a telemeter system in copending application Ser. No. 231,702 filed July 15, 1975.
On the other hand, such a system using a sound pulse signal has a defect that it may be distributed by an echo signal. That is, besides a sound pulse signal transmitted straight from a transmitting transducer of a remote measuring device in sea water to a receiving transducer, because of wireless transmitting the radiated sound pulse signal is reflected from various objects such as the shore and the bottom and the surface of the sea and a so-called echo signal is received and causes an error in measurement. Therefore, it is necessary to eliminate the echo signal or to compensate for such an undesirable effect of the echo signal while measuring the pulse spacing, which corresponds to information to be measured, of the received sound pulse signal.
In the prior art, for this purpose there is used an ultrasonic receiver having a very sharp directivity so as to pick up only the signal transmitted straight from the radiating source. However, when the measuring probe moves for example for measuring temperature at various depths, it is required that the receiver always trace the probe correctly and this is almost impossible in practice. Besides, as an electronic signal processing method for eliminating the echo signal, it has been considered to clamp the receiving operation of the receiver for a certain period after arrival of a sound pulse signal. However, under conditions where temperature changes greatly, the pulse spacing of the sound pulse signal also changes greatly and it is almost impossible to set on optimum clamping period. Further, although there has been considered a correlation method of distinguishing the normal signal from the echo signal, this results in a very complex system and is not practical.
Moreover, there is another problem in connection with the measuring probe. That is, it is inevitable that the detecting characteristic of one sensor is different from that of another sensor. For example, when thermistors are used as sensors for detecting temperature, the manner in which the resistance of one of the thermistors changes with a temperature change is not always the same as the manner in which another transistor of the thermistors changes with the same temperature change. Although such deviation can be compensated for by special design of an electronic circuit, it is not desirable, mainly from the point of cost, to add an accurate and complex compensating means to the expandable probe. Therefore, for telemetering information relating to ocean water with use of an expendable probe, usually by measuring the detecting characteristic of each sensor, the sensors having almost the same characteristic are selected and mounted on the probes. Alternatively, by using a calibration chart prepared for each probe, the correct value is obtained for the received signal.